The present invention relates to a low pressure fuel circulation with fuel pre-heating for an air-compressing injection internal combustion engine, especially for commercial vehicles.
It is known that in the low pressure fuel circulation for internal combustion engines with diesel operation the fuel is sucked-in by a pump initially by way of relatively coarse-meshed filters and is conducted by way of a fine filter to the injection pump and cleansed. However, diesel fuels have the property at low temperatures (those under the so-called BPA-point) to separate out or precipitate paraffin crystals which clog up the filter and lead to operating troubles of the vehicle.
The not-yet-published German Patent Application No. P 35 38 360.7-13 proposes as effective remedial measure, that the fuel is preheated by a heat-exchanger operatively connected with the cooling water circulation and is fed back in a low viscous condition.
Such a fuel pre-heating is satisfactory for passenger motor vehicles; however, pure water-fuel heating systems are not suitable for commercial vehicles to avoid the clogging-up of the fine filter prior to reaching a sufficiently high heating water temperature by reason of the low waste heat of directly injecting engines and in view of the as a rule large water content of large-volume commercial-type vehicle engines.
Furthermore, the use of electric pre-heating systems is known from the DE-OS No. 30 34 730 which, however, entail the disadvantage that the battery capacity of the vehicle is subjected to additional loads to a not insignificant degree and a relatively high expenditure is required also by the cabling requirement.
The present invention is therefore concerned with the task to undertake simple measures to assist in avoiding the operating troubles of the internal combustion engine which, however, counteract in particular the short intermediate condition between the response of the water-heating system, respectively, of the heat-exchanger cooperating with the cooling circulation and the clogging of the fine filter.
The underlying problems are solved according to the present invention in that the inlet line is provided with a by-pass by-passing the fine filter which includes an excess pressure valve opening in dependence on a predetermined maximum pressure acting on the fine filter as a result of paraffin formation. Owing to the arrangement of an excess pressure valve in a by-pass by-passing the fine filter, the fuel initially flows into the filter housing before it can reach the injection pump when reaching the opening pressure as a result of paraffin accumulation. The warming-up fuel dissolves the paraffin accumulation little-by-little and reduces the difference pressure at the filter which finally leads to the closing of the excess pressure valve.
As the excess pressure valve not only responds to paraffin accumulation at the filter input but also to contaminations, which cause the opening of the excess pressure valve whereas this cannot be recognized as such by the driver, the volume stream reaching the injection pump can be limited according to a further feature of the present invention by the arrangement of a throttle in the by-pass downstream of the excess pressure valve, whereby the clogging of the filter will be noticed by the driver by the decrease of the engine output.
A further advantageous feature of the present invention resides in providing in the by-pass a pressure monitoring device with a signal transmitter, by means of which the clogged-up filter is indicated to the driver.
According to still further features of the present invention pursuant to which the excess pressure valve is thermostatically controlled, one obtains the advantage that the excess pressure valve only opens when the fuel pressure increases correspondingly ahead of the filter by paraffin formation but locks automatically when the warm operating temperature adjusted by the thermostat is reached by the heat-exchanger. The phase of the passage of unfiltered fuel by way of the by-pass can be controlled more precisely thereby. The construction of apertured disks serves to circumcirculate the thermostat well with fuel when the excess pressure valve is opened and thus to improve the response behavior.